After a borehole has been formed downhole into the earth and the casing cemented into place, the well must be completed by lowering a perforating gun downhole into a position adjacent to a hydrocarbon-containing formation. There may be several different elevations or pay zones to be perforated with the gun, and accordingly, it is often necessary to be able to relocate the gun several different times in order to perforate all of the hydrocarbon-containing formations.
In the past, it has been found to be difficult to detonate a large wireline operated casing type perforating gun in a dry hole because the tremendous energy released by the gun thrusts the gun uphole and entangles the gun with the wireline, thus necessitating a costly fishing job. For this reason, it is customary to fill the wellbore with a liquid, thereby dampening this upthrust action when the perforating gun is detonated. However, there are many instances where it is advantageous to perforate a dry hole as opposed to a wet hole. This is especially so where it is advantageous to develop a maximum pressure differential between the formation and the interior of the casing at the moment the casing is perforated. Such a situation is frequently encountered with respect to old wells that are no longer producing or have never produced, and are cased down to the pay zones but contain no tubing or other piping in the wellbore, for example. Since such wells may not be capable of production even after perforation of all the potential pay zones, the more costly and complicated techniques and apparatus employed with wet hole perforating practices are often not practical or justified. Perforation with a wireline operated casing type gun in these old, dry wells, on the other hand, is relatively efficient and economical, thereby avoiding one of the primary obstacles in the way of putting such wells in useful service.
Various devices and methods have been tried in the past to secure a wireline operated casing type perforating gun to the piping to prevent upthrust of the gun upon detonation, but these prior art approaches have been unsatisfactory because they do not enable the gun to be securely anchored prior to detonation, but instead are actuated by the detonation itself, when the gun is being thrust upwardly with great force. This upward thrust of the gun sometimes prevents the prior art anchoring devices from securely engaging the piping, thereby resulting in the gun being thrust uphole with the accompanying wireline entanglement problems and subsequently required fishing job referred to above.
Accordingly, it would be desirable to provide a wireline operated jet perforating gun with anchoring means by which the gun could be safely and releasably anchored downhole in a borehole prior to detonation of the gun in a manner to prevent upthrust of the gun when the gun is detonated. It would furthermore be desirable to be able to relocate a select fire perforating gun any number of times so as to achieve perforation of a plurality of different hydrocarbon-containing formations in a single trip into a dry wellbore.
Apparatus and method which attains the above desirable goals is the subject of the present invention.